There currently exist small sensing devices that can be worn by a user to monitor physical activity performed by the user. As an example, the FitLinxx® ActiPed+ (available from FitLinxx, Shelton, Conn., USA) is a small device that can be clipped to a shoe and used to monitor walking and running activities by the user. When a user walks or runs, an on-board accelerometer outputs data that is stored on the device for subsequent transmission to a computer system. The computer system can analyze the data to determine activity type, and calculate various activity parameters (e.g., duration of activity, total steps, distance traveled, and calories burned). Results of the data analysis may be presented on the computer's display, so that a user may review details of his or her activity. The results may be stored, so that the user can maintain a record of exercise regimens and track progress toward exercise goals, or so that the data may be used by medical personnel to track recovery from an illness or injury. Other modern activity monitors perform similar functions with varying degrees of accuracy.
Most activity monitors are configured to be attached to a subject's clothing or strapped to a subject's limb. For example, some activity monitors may clip on clothing, or be configured to clip on or lace in a shoe. Activity monitors that attach to clothing are generally not adapted to sense a physiological parameter of the subject. Some activity monitor's that may be worn on the wrist or ankle of a subject may be adapted to sense heart rate, but these monitors generally cannot measure details of cardiac waveform to obtain information such as heart-rate variability (HRV) or cardiac abnormalities such as arrhythmias.